Given the size of the present front teeth... I think I'm gonna go with a missing lateral incisor that never even developed, giving the left central incisor too much room that allowed it to shift to the left and grow in at abnormal placement. Since that one next to the fang is comparatively too large to be the smaller lateral incisor that should be there.

Considering how rough Raph could be with his younger brothers in the beginning he could be the reason for the missing tooth. And that could’ve the reason Donnie was so quick at caving to Raph’s threats of hitting him.

I wonder when they decided it was a missing tooth and not a gap, cause concept art say gap and the OotS game have Donnie depicted with a gap. Possibly a decision made for the Monster arc as that's when we see the Nick design Turtles with individual sculpted teeth on the cgi model.

Although the designs from OotS the Game can be easily chalked down to communication issues or Red Fly's own design decision like revoking the third frontal toe